GOP Sen. Lamar Alexander won't seek reelection

Sen. Lamar AlexanderAndrew (Lamar) Lamar AlexanderThe Hill's Morning Report - Presented by JUUL Labs - Trump attack on progressive Dems draws sharp rebuke Republicans make U-turn on health care Trump announces, endorses ambassador to Japan's Tennessee Senate bid MORE (R-Tenn.) on Monday announced that he will not run for reelection in 2020.

"I will not be a candidate for re-election to the United States Senate in 2020. The people of Tennessee have been very generous, electing me to serve more combined years as Governor and Senator than anyone else from our state," Alexander said.

Alexander, who has been in the Senate since 2003, said he will serve out the remainder of his term, which runs through the end of 2020. 

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"I have gotten up every day thinking that I could help make our state and country a little better, and gone to bed most nights thinking that I have. I will continue to serve with that same spirit during the remaining two years of my term," he added.

Alexander, 78, is the first senator up in 2020 to announce that they won't seek reelection. Republicans face a challenging map during the next cycle, where they'll be defending approximately 22 seats, including in states like Colorado and Maine, which Democratic presidential nominee Hillary ClintonHillary Diane Rodham ClintonHillary Clinton slams Trump rally: 'The time has come again' to fight for democracy Trump blasts minority Democrats, rally crowd chants 'send her back' The Memo: Democrats debate Trump response – 'Being righteous and losing sucks' MORE won in 2016.

Alexander is also the latest in a string of GOP chairmen from the establishment wing of the party to announce that they will retire during the Trump era: Senate Finance Committee Chairman Orrin HatchOrrin Grant HatchTrump to award racing legend Roger Penske with Presidential Medal of Freedom Trump awards Presidential Medal of Freedom to economist, former Reagan adviser Arthur Laffer Second ex-Senate staffer charged in aiding doxxing of GOP senators MORE (R-Utah) and Senate Foreign Relations Committee Chairman Bob CorkerRobert (Bob) Phillips CorkerTrump announces, endorses ambassador to Japan's Tennessee Senate bid Meet the key Senate player in GOP fight over Saudi Arabia Trump says he's 'very happy' some GOP senators have 'gone on to greener pastures' MORE (R-Tenn.) are retiring at the end of the current Congress.

Corker, on Monday, called working with Alexander "one of the highlights" of his work in the Senate.

"As one of the finest statesmen our state has ever seen, Lamar will leave behind a remarkable legacy. I know he will press through the next two years with great vigor, and I look forward to all he will accomplish on behalf of Tennesseans as he completes his service in Washington," Corker said in a statement.

Alexander previously served as chairman of the Senate Republican Conference and is considered to be close to both Majority Leader Mitch McConnellAddison (Mitch) Mitchell McConnellMcConnell: Trump 'on to something' with attacks on Dem congresswomen Dems open to killing filibuster in next Congress Senate passes bill making hacking voting systems a federal crime MORE (R-Ky.) and Minority Leader Charles SchumerCharles (Chuck) Ellis SchumerTop Democrats demand security assessment of Trump properties Lawmakers pay tribute to late Justice Stevens Trump administration denies temporary immigrant status to Venezuelans in US MORE (D-N.Y.).

He's also been at the center of some of the biggest fights in the Trump administration because of his position as chairman of the Health, Education, Labor and Pensions (HELP) Committee. 

He was part of a working group of GOP senators, assembled by McConnell, that crafted the Senate GOP's failed ObamaCare replacement bill last year. 

But Democrats have credited him with being willing to work with them on key issues despite growing partisanship in the Senate. 

He crafted a deal with Sen. Patty MurrayPatricia (Patty) Lynn MurrayTrump's new labor chief alarms Democrats, unions Overnight Health Care — Presented by PCMA — Sanders mounts staunch defense of 'Medicare for All' | Biden, Sanders fight over health care heats up | House votes to repeal ObamaCare 'Cadillac Tax' | Dems want details on fetal tissue research ban Democrats demand information from White House about fetal tissue research ban MORE (D-Wash.), the top Democrat on the health committee, to funds key payments to health insurers for two years.

He was also one of less than 10 GOP senators who voted for a bipartisan immigration proposal earlier this year that would have provided approximately 1.8 million immigrants brought to the United States illegally as children an eventual pathway to citizenship in exchange for $25 billion in border security. 

Sen. Ed MarkeyEdward (Ed) John MarkeyDemocrats warm to idea of studying reparations Hillicon Valley: Senators unload on Facebook cryptocurrency plan | Trump vows to 'take a look' at Google's ties to China | Google denies working with China's military | Tech execs on defensive at antitrust hearing | Bill would bar business with Huawei Senators press FTC over 'woefully inadequate' Facebook settlement MORE (D-Mass.) on Monday also credit Alexander as being a "true partner" in addressing the country's opioid epidemic. 

Alexander's decision to retire will leave a hole in Tennessee politics, where he has been a mainstay for decades. He was first elected to the Senate in 2002, previously served as governor of Tennessee and made two unsuccessful bids for the White House in 1996 and 2000.

But he was considered potentially vulnerable to a primary challenge in 2020 after winning narrowly fending off tea party challenger Rep. Joe Carr during the 2014 GOP primary. 

It's unclear who will vie to succeed Alexander. Tennessee Gov. Bill Haslam (R) flirted with a Senate bid in 2018. And state Sen. Mark GreenMark GreenWHO says Ebola is global emergency Trump's family separation policy has taken US to 'lowest depth possible,' says former immigration lawyer Four heated moments from House hearing on conditions at border facilities MORE (R) hinted last year that he could make a Senate run in 2020.  

Green told USA Today in August that he would be "back in the election cycle" in 2020.

"I think that's going to be an opportunity for me potentially later," he added about a potential Senate run. 

President TrumpDonald John TrumpAmash responds to 'Send her back' chants at Trump rally: 'This is how history's worst episodes begin' McConnell: Trump 'on to something' with attacks on Dem congresswomen Trump blasts 'corrupt' Puerto Rico's leaders amid political crisis MORE won Tennessee by more than 26 points in 2016. In last month's election to replace Corker, Rep. Marsha BlackburnMarsha BlackburnSocial media summit highlights partisan approaches on tech Trump announces, endorses ambassador to Japan's Tennessee Senate bid Hillicon Valley: Trump rails against 'terrible bias' at White House social media summit | Twitter hit by hour-long outage | Google admits workers listen to smart device recordings MORE (R-Tenn.) defeated former Gov. Phil Bredesen (D) by nearly 11 points.

Updated at 1:12 p.m.